The present invention concerns a valve arrangement for adjusting a hydraulic saw chain tightening device in sawing gear that includes a holder and a saw bar displaceable in a longitudinal direction relative to the holder for regulating the tightness of a saw chain running around the saw bar and holder, whereby the saw chain tightening device is connected by the valve arrangement to a hydraulic circuit system and includes at least one hydraulic piston affecting the saw bar.
When mechanised means are used to improve the efficiency of logging operations, it is preferable to use what is known as a timber cutting unit comprising a sawing gear that includes a saw chain and a saw bar. Efficient logging puts high demands on the equipment used with regard to safety, robustness, ease-of-use as well as precision and performance, i.e. that the equipment works quickly.
It is well known that a saw chain that rotates under conditions of high speed around a sew bar requires chain tightening at relatively short intervals. In addition, it is also known that the problem of chain tightening can be solved by an automatic and continuous tightening of the saw chain, which is achieved by making the saw bar displaceable in the saw bar's longitudinal direction in relation to the drive gear of the saw chain. However, the chain-tightening force must not be too great, as too great a tension increases the wear on the chain and shortens the working life of both the saw chain itself and of the wheel around which the saw chain runs at the free end of the saw bar.
An attachment for a saw bar of a chain saw is described in SE 467,488. The attachment includes plate with a grooved recess in which a steering part united with the saw bar moves in the saw bar's longitudinal direction. A piston projecting from the recess in the plate in the longitudinal direction of the saw bar bears on the guiding part when in its active operating position. The piston gets its power from a fluid system integrated and enclosed in the plate and that includes a gas-sprung pressure chamber that is connected to the piston by a line and a valve. The valve is a manually switchable check valve, that in one position permits fluid to flow only from the pressure chamber to the piston, thus tightening the chain by displacing the saw bar, and in a second position permits fluid to flow only from the piston to the pressure chamber, which, due to an increase of pressure in the chamber, restores the position of the piston when, for example, replacing the saw chain with a new one.
The technology of an enclosed system carries with it, among other things, the limitation that the pressure at the piston varies depending on the position of the piston. The stretching force is greater when the piston is in its inner position and somewhat less when the piston is in its outer position. Further disadvantages are the problems of having to maintain and/or consciously be able to vary the pressure and thus the stretching force over time.
When a new chain is to be fitted, the check valve is switched and a levering tool is used to force back the piston. A high pressure in the enclosed hydraulic system, which is needed to ensure a sufficiently great stretching force for the saw chain, means that a powerful force is required to push back the piston. This is unsatisfactory, partly because of the working environment and the risk of injury to the user, and partly because loose specialised tools have a tendency to disappear or be difficult to find when they are needed.
SE 502,386 illustrates how a similar piston is arranged, in this case in a steering part permanently attached to the saw bar and accommodated in a groove in the attachment/holder, whereby the piston, when affected by the fluid under pressure, keeps the saw bar in a position displaced from the attachment/holder. In this case, the piston receives its power from a hydraulic system on the mechanical equipment on which the sawing gear is mounted. An additional pair of pistons are also found on the same hydraulic system arranged to stabilise the saw bar by means of a clamping action across the direction of displacement of the saw bar. The saw bar is thus affected partly by a piston arranged to push the blade from the holder so that the saw chain is kept tight, and partly by at least one clamping piston that holds the blade tightly and stabilises it in position. The force keeping the chain tight is somewhat greater than the force of friction that results from the clamping pistons so that a continuous tightening is achieved while the chain still displays a relatively stable attachment. All the pistons are connected to the machinery's hydraulic system and when this equipment is in operation, the pistons are provided with what is essentially a constant pressure.
A check valve is also arranged in the line from the hydraulic system. This check valve is essential to ensure that the saw chain does not become slack during operation. This could happen as a result of the rapid loading and pressure increases that occur at the piston, for example, at start-up when the saw chain goes from stationary position to full speed in a very short time, or when a saw chain moving a high speed is forced against a tree trunk or when a branch gets caught in the chain, etc.
When a saw chain is to be replaced, the pistons must be forced back, which means that the hydraulic fluid behind the pistons must be disposed of. In devices similar to that described in SE 502,386, this is done as follows. Firstly, the machinery is stopped so that the hydraulic system loses its pressure. The check valve, however, ensures that the pressure is maintained behind the piston(s), which is why chain replacement calls for emptying to ease the pressure. In practice, emptying requires that the operator needs to have a special tool at hand to open the evacuating valve that is normally located on the sawing gear. As a collection vessel for the fluid is usually not available, this evacuation means an unwanted oil spill in a natural environment. In addition, evacuation also brings the risk that air may enter the hydraulic system, which if it happens, will greatly diminish the sought after rigid tightening and stabilising function of the pistons.